1. Field of the Invention
The present invention relates to a back pressure control apparatus for a fuel cell system.
2. Description of the Related Art
A typical example of solid polymer electrolyte fuel cells has a membrane electrode assembly in which an anode and a cathode are provided on either side of a solid polymer electrolyte membrane. Each electrode assembly is placed between a pair of separators so as to support the electrode assembly and form a planar unit cell, and generally, a specific number of the unit cells are stacked in the direction of thickness, so as to obtain a fuel cell stack.
In each unit cell, a fuel gas passage through which a fuel gas (e.g., hydrogen) passes is formed on a surface of an anode separator, that is, one of the separators which faces the anode, and an oxidizing gas passage through which an oxidizing gas (e.g., air including oxygen) passes is formed on a surface of a cathode separator, that is, one of the separators which faces the cathode. In addition, a coolant passage through which a coolant (e.g., cooling water) passes is formed between a separator of a unit cell and a separator of another unit cell which is adjacent to the former unit cell.
Generally, the oxidizing gas is supplied to the oxidizing gas passage by activating a supercharger which is connected to the oxidizing gas passage, and opening a back pressure control valve which is attached to the downstream of the oxidizing gas passage with respect to the fuel cell. That is, the flow rate and the pressure of the oxidizing gas supplied to the fuel cell are controlled according to the rotation speed of the supercharger and the degree of opening of the back pressure control valve.
The back pressure control valve is, for example, a butterfly valve, which is attached to an oxidizing gas passage. This oxidizing gas passage passes through a fuel cell which includes a plurality of unit cells, where the unit cells are stacked in a horizontal direction, and thus the oxidizing gas passage passes through the unit cells in the stacking direction.
When hydrogen, which functions as a fuel gas, is supplied to a reaction plane of the anode, hydrogen is ionized and the hydrogen ions are transferred to the cathode via a solid polymer electrolyte membrane. During this process, electrons are generated and flow to an external circuit, providing DC (direct current) electric energy. Here, air is supplied as an oxidizing gas to the cathode, and the hydrogen ions, electrons, and oxygen in the air react at the cathode, thereby producing water.
Such water (called xe2x80x9cproduced waterxe2x80x9d hereinbelow) is produced mainly in an oxidizing gas passage in the fuel cell. When the fuel cell system is stopped and is maintained in a low-temperature environment, the above-explained produced water freezes; thus, it is difficult to restart the fuel cell system. Therefore, while the fuel cell system is stopped, the produced water should be removed through each corresponding passage of the fuel cell system.
If the oxidizing gas passage, which has a relatively small cross-sectional area (through which the fluid passes), receives a great amount of the oxidizing gas supplied from the supercharger, a high flow velocity can be obtained and the produced water can be effectively drained. However, in the vicinity of the back pressure control valve which is attached to the end of the oxidizing gas passage, the cross-sectional area of the passage is relatively large. Therefore, sufficient flow velocity cannot be obtained and a part of the produced water discharged from the fuel cell may remain.
If the produced water remains in the vicinity of the back pressure control valve and the fuel cell system is maintained in a low-temperature environment after the system is stopped, then the remaining water is frozen and lumps of ice are produced. Therefore, when the system is restarted, the lumps of ice contact the valve element of the back pressure control valve, so that the normal operation of the back pressure control valve may be blocked.
In consideration of the above circumstances, an object of the present invention is to provide a back pressure control apparatus (typically, a back pressure control valve) used in a fuel cell system, which can normally operate even when the system starts in a low-temperature environment.
Therefore, the present invention provides a back pressure control apparatus in a fuel cell system, which is connected to an oxidizing gas passage, wherein the connection position is downstream relative to a fuel cell, the back pressure control apparatus comprising:
a pipe member which functions as a portion of the oxidizing gas passage and is arranged along an approximately horizontal direction, where a predetermined area of the inner surface of the pipe member is subjected to a hydrophilic process in advance; and
a valve element which is provided in the pipe member and is rotatable around a shaft, where the shaft is arranged in a cross-sectional direction of the pipe member.
According to the above back pressure control apparatus which is positioned at the downstream relative to the fuel cell and has a pipe member arranged along an approximately horizontal direction, the overall fuel cell system can have a low profile.
When the back pressure control apparatus having the above structure is opened and an oxidizing gas is supplied to the oxidizing gas passage of the fuel cell, water produced in the fuel cell is transferred through the oxidizing gas passage and is drained via the back pressure control apparatus. In this process, the produced water may remain in the vicinity of the back pressure control apparatus due to the horizontally-arranged form of the pipe member. However, the inner surface of the pipe member is subjected to a hydrophilic process in advance; thus, the surface tension of each water drop which contacts the processed inner surface is decreased, so that the contact angle between the surface of the pipe member and the tangential plane of the water drop is small.
Therefore, even if some water drops remain in the vicinity of the back pressure control apparatus due to insufficient draining of the produced water and these water drops freeze, the frozen water drops do not have a height (in the radial direction of the pipe member) which will cause the frozen water drops to contact the valve element, which is provided for closing the pipe member when the operation of the fuel cell is restarted. Therefore, it is possible to prevent the rotation of the valve element from being blocked.
Accordingly, even if the fuel cell is restarted in a low-temperature atmosphere, optimum flow-rate control can be performed. In addition, no external device, such as a heater, for defrosting frozen water drops is necessary; thus, the starting performance of the fuel cell in the low-temperature atmosphere can be improved by employing a simple structure which is low in cost.
The present invention also provides a back pressure control apparatus in a fuel cell system, which is connected to an oxidizing gas passage, wherein the connection position is downstream relative to a fuel cell, the back pressure control apparatus comprising:
a pipe member which functions as a portion of the oxidizing gas passage and is arranged along an approximately vertical direction, where a predetermined area of the inner surface of the pipe member is subjected to a water repellent process in advance; and
a valve element which is provided in the pipe member and is rotatable around a shaft, where the shaft is arranged in a cross-sectional direction of the pipe member.
According to this back pressure control apparatus which has a pipe member arranged along an approximately vertical direction, gravity acts downwardly on water drops which remain on the inner surface of the pipe member, In addition, the inner surface of the pipe member is subjected to a water repellent process in advance; thus, the surface tension of each water drop which contacts the processed inner surface is increased, thereby increasing the contact angle. As a result, the contact area between the water drop and the inner surface of the pipe member is decreased, so that the water drop tends to fall along the inner surface of the pipe member due to gravity. Accordingly, it is possible to prevent lumps of ice from being formed in the operational range of the valve element which is rotated when the operation of the fuel cell is restarted.
Therefore, even when the water drops freeze at low temperatures, the frozen water drops do not interfere with the valve element. Accordingly, even if the fuel cell is restarted in a low-temperature atmosphere, optimum flow-rate control can be performed
In either type of the back pressure control apparatus, the diameter of the valve element may be less than the inner diameter of the pipe member.